An arm-type unloader wheel as described in my U.S. Pat. No. 4,320,828 is held under a dome in the bottom of a bulk-storage bin, generally above a discharge port of same. The wheel is rotated so its arm or arms engage out past the dome into the bottom of the mass of bulk material in the bin, loosening this material so it flows, and simultaneously sweeping it into the discharge port. Two such wheels can be provided flanking the discharge port, or this port can be positioned eccentrically underneath a single wheel.
In German patent document No. 2,364,830 (and corresponding U.S. Pat. No. 3,927,774) there is described a system wherein the unloader wheel is carried on a vertical shaft provided below the wheel with a hook-tooth sprocket that cooperates with a pair of drive units each including a double-acting hydraulic cylinder having an outer end pivoted about a vertical axis on the machine frame and an inner end carrying a pusher element engageable behind the teeth of the sprocket. These inner ends are biased radially inward so the pusher elements engage the sprocket's periphery.
In this earlier system the cylinders were expanded and contracted alternately so that the one cylinder would be expanding while engagement with the sprocket to rotate the wheel while the other cylinder was contracting to get back into a starting position. Such style of operation on the one hand leads to irregular displacement of the unloading arm or arms, sometimes fast and sometimes slow. In addition it exerts considerable radial forces on the shaft of the unloader so that same must be very massively constructed and mounted in extremely robust bearings. Furthermore it is necessary to use a very powerful actuator in order to break the arm free at the start of operation, even though once the unloader has started it can be driven with much less force.